We report a study of the plasmonic properties of a 20-nm-thick nanoporous Au film by far-field and near-field optical techniques. The film is prepared sequentially by deposition of gold and copper, thermal annealing, and chemical etching, and has randomly distributed nanopores with sizes ranging between 20 and 350 nm. The absorbance of the nanoporous Au film is much higher than that of a plain Au film and can be attributed to the conversion of incident light into surface plasmon polaritons (SPPs). In addition, a broad peak appears at around 630 nm in the scattering spectrum and serves as evidence of hole plasmon resonance. From transmission mode near-field scanning optical microscopy measurements, two types of local field enhancement are observed. One has a small spatial extent of around 200 nm and the other has a large spatial extent of around 1 μm. The two types of enhancement correspond to strong and weak SPP localizations, respectively.

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